The voltage of a single galvanic cell varies between about 0.5 and 3.0 volts depending on active materials and remaining capacity. To achieve higher voltages two or more cells must be connected in series.
A very usual problem in battery systems comprising several cells in series is that when one single cell is weaker than other cells, at the end of the discharge it may reverse its polarity and become permanently damaged. The whole power source is weakened and its theoretical energy is thus only partially utilized. This problem is well known with nickel-cadmium batteries. NiCd batteries can be charged and discharged up to 1000 times as single cells. Depending on the discharge current and the number of cells in series, the cycle life of a deep-discharged NiCd battery can be less than twenty. Also the voltage of most batteries varies depending on the state of charge and discharge current which sometimes is a problem.
Connecting many cells in series is not economical and results in lower energy density when compared to filling the available space with a few cells or preferably with just one large cell. The room for batteries in an electrical device is determined by the required voltage and the size and shape of available batteries. A typical shape is cylindrical, which means that in multicell batteries much of the available space cannot be utilized.
Some electrical devices have been developed to solve the problems discussed above. Common to all of them is to electrically raise the voltage of one cell or a few cells to a higher level.
U.S. Pat. No. 4,121,115 presents a power source having a battery as a primary source and a step up type voltage converter which keeps the output voltage constant. U.S. Pat. No. 4,085,358 shows converters which produce about 8 volts from input voltages of 2.0 to 3.3 volts. Both circuits utilize bipolar transistors which work properly, even at low voltages, but the voltage drop across these transistors causes the efficiency of the circuits to be comparatively low.
Swedish Patent No. SE B 444626 discloses a combination of a lithium cell and a transformer based device which raises the voltage to between 6 and 500 volts. Because of the transformers, the output voltage is proportional to the input voltage and voltage adjusting can be made only by changing the number of wire turns in the transformer. Bipolar transistors in this device also cause energy losses such that high efficiency cannot be achieved. In addition, a transformer type device is comparatively heavier and bigger than other types of DC-DC converters.
German Patent No. DE A1 3610035 describes a device utilizing a MOSFET transistor, which makes it efficient even at low primary voltages. A disadvantage is that, because of this transistor, the device must have at least 4 volts as a start voltage.